1. Field of the Invention
The present invention relates to a method for producing a bonded SOI substrate utilizing a silicon single crystal wafer produced by the Czochralski method (referred to as the "CZ method" hereinafter), and a bonded SOI substrate. In particular, the present invention relates to a method for producing a thin film SOI substrate having an SOI layer thickness of 1 .mu.m or less, wherein the size of crystal defects called grown-in defects in the SOI layer is made smaller, and therefore the defects can be easily eliminated by means of heat treatment, and an SOI substrate produced by the method.
2. Description of the Related Art
As a technique for bonding two pieces of silicon single crystal wafers with an interposed silicon oxide layer, there has been known a method disclosed in, for example, Japanese Patent Application Publication (KOKOKU) No. 5-46086, which comprises forming an oxide layer on at least one wafer, closely contacting them with each other while eliminating any foreign substances from the bonding surfaces, and subjecting them to a heat treatment at a temperature of about 200-1200.degree. C. to enhance the bonding strength.
Because a bonded substrate whose bonding strength has been enhanced by a heat treatment can subsequently be processed by, for example, grinding, polishing or the like, either one of the wafers can be made thinner to a desired thickness by grinding, polishing or the like to afford an SOI layer on which devices are formed.
That is, because the produced SOI substrate contains bulk crystals of the silicon single crystal wafer exhibiting excellent crystallinity as they are, the obtained SOI layer would exhibit excellent crystallinity, and this characteristic is more advantageous compared with other SOI production methods such as the SIMOX (Separation by Implanted Oxygen) method and the fusion recrystallization method.
On the other hand, as the starting material of bonded substrates, i.e., silicon single crystal wafers, those silicon single crystal wafers obtained by slicing a single crystal ingot grown by the Czochralski method (CZ method) have been mainly used.
However, it has recently been reported that, in silicon single crystals grown by the CZ method, crystal defects introduced during the crystal growth and called grown-in defects as mentioned above were detected by various measuring methods. For example, as for single crystals pulled at a growth rate generally used for the commercial production (for example, about 1 mm/min or more), these crystal defects can be detected as pits by preferentially etching (Secco etching) their surfaces with Secco solution (mixed solution of K.sub.2 Cr.sub.2 O.sub.7, hydrofluoric acid and water). See Japanese Patent Application Laid-open (KOKAI) No. 4-192345.
The major cause of these pits is considered to be clusters of vacancies, which aggregate during the production of single crystals, or oxide precipitates, which are aggregates of oxygen atoms contaminated from quartz crucibles. Because these crystal defects present in regions where devices are to be formed may become harmful defects that degrade device characteristics, various kinds of methods for decreasing such crystal defects have been investigated.
For example, it has been known that, in order to reduce the density of the aforementioned clusters of vacancies, the crystals may be grown at an extremely low growth rate (for example, 0.4 mm/min or less). See Japanese Patent Application Laid-open (KOKAI) No. 2-267195. However, it has been becoming clear that this method generates crystal defects that are considered dislocation loops formed by newly gathered excessive interstitial silicon, and they markedly degrade the device characteristics, which means that the method cannot be a solution of the problem. In addition, such a decrease of crystal growth rate from the conventional 1 mm/min or more to 0.4 mm/min or less invites marked decrease of the productivity and increase of the cost of single crystals.
Therefore, if an SOI substrate is produced by using a CZ wafer containing such crystal defects, the produced SOI layer of course contains crystal defects, and they degrade the electric characteristics of the SOI layer such as oxide dielectric breakdown voltage. In particular, when the substrate is a thin film SOI substrate, i.e., the SOI layer to be formed must have a thickness of, for example, 1 .mu.m or less, the crystal defects may be present in such a manner that they penetrate the SOI layer and thus form pin holes, which markedly degrade the quality and the characteristics.